The subject application is related to subject matter disclosed in Japanese Patent Application No. Hei 11-67513 filed on Mar. 12, 1999 in Japan to which the subject application claims priority under the Paris Convention and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device such as a DRAM or an SRAM having a redundancy circuit, a semiconductor integrated circuit having a redundancy circuit, and a method of manufacturing a semiconductor device having a redundancy circuit. In particular, the present invention relates to the structure of a fuse to be blown by a laser beam to realize a relief function.
2. Description of the Related Art
Some semiconductor integrated circuits are provided with a redundancy function to cover a defective part. The redundancy function is realized by a redundancy circuit. The redundancy circuit in a DRAM or an SRAM replaces defective memory cells with redundant memory cells that arranged in a memory cell matrix. To generate a signal for replacing defective memory cells with redundant memory cells, fuses related to the redundancy circuit are blown by laser beams.
FIG. 1A is a partly-cut top view showing fuses according to a prior art, FIG. 1B is a sectional view taken along a line Ixe2x80x94I of FIG. 1A, and FIG. 1C is a sectional view taken along a line IIxe2x80x94II of FIG. 1A. To guide a laser beam for blowing a required fuse 2, an opening 4 is formed in a polyimide film 1. Each fuse 2 runs across the opening 4. Peripheral circuits including a redundancy circuit related to the fuses 2 are arranged around the opening 4. In FIG. 1B, each fuse 2 is formed on an insulating layer 9. On the fuse 2, a passivation layer 7 is formed. Under the opening 4, there are only the fuses 2, and no connection parts such as plugs or wiring are present under the opening 4. In FIG. 1C, a given fuse 2 is cut by irradiating it with a laser beam. The laser beam forms a blown part 5 to disconnect the fuse 2, when replacing a defective line with a redundant line.
According to the prior art, a pitch between adjacent fuses is determined by the diameter of a laser beam to blow each fuse, and therefore, an only way to reduce an area where the fuses are formed is to reduce the diameter of a laser beam. According to the prior art, each fuse involves circuits including a redundancy circuit at both ends of the fuse, and therefore, the periphery of the opening 4 for the fuses is crowded with such circuits to hinder miniaturization.
An object of the present invention is to provide a semiconductor device having a small fuse area with fuses densely arranged therein and peripheral circuits arranged on one side thereof.
Another object of the present invention is to provide a semiconductor integrated circuit having small fuses and fuse-related circuits on one side of each fuse.
Still another object of the present invention is to provide a method of manufacturing a semiconductor integrated circuit having small fuses and fuse-related circuits on one side of each fuse.
In order to accomplish the objects, a first aspect of the present invention provides a semiconductor device having a first conductor, a columnar second conductor having a bottom face that is in contact with a top face of the first conductor, a first insulating film for covering the first and second conductors, a linear third conductor formed on the first insulating film and having an end whose bottom face is in contact with a top face of the second conductor, and a second insulating film for covering the third conductor and first insulating film and having a thin area over the second conductor.
The thin area corresponds to an opening for guiding a laser beam and is formed by removing a polyimide film or by thinning a passivation film so that a fuse including the second and third conductors may easily be blown by a laser beam. This structure provides the following effects when a laser beam is emitted toward a contact face between the second and third conductors:
(1) The wavelength of the laser beam is set to the light absorbing band of the third conductor so that only the third conductor may be heated and blown without damaging the other conductors. To also blow the second conductor, the same material may be used for the second and third conductors, or the wavelength of the laser beam may be adjusted.
(2) The second and third conductors block the laser beam to reach the first conductor, thereby preventing the heating or damaging of the first conductor.
(3) The second conductor absorbs heat from the insulating films, to heat the second conductor itself. This makes the removing of the second conductor easier and suppresses the heating and damaging of the first conductor.
(4) The second conductor may be made from a material having low heat conductivity, or may be shaped to show large heat resistance, to suppress heat conduction in the second conductor. This may prevent the heating and damaging of the first conductor even if the temperature of the third conductor is increased. The material having low beat conductivity is, for example, tungsten, titanium, tantalum, and silicon. The second conductor may include a plurality of plugs having a laminated structure, or may have a shape of large aspect ratio. If the first conductor must be removed also, the first conductor may be made of a material having high heat conductivity, such as aluminum and copper.
(5) The second conductor may be made of a material having high melting and boiling points, to prevent the deformation of the second conductor and keep the shape of the first conductor. The material having high melting and boiling points is, for example, tungsten, titanium, tantalum, and silicon. To remove the second and first conductors, the material may be aluminum.
(6) After a shot of laser beam, the first insulating film secures an insulating distance corresponding to the length of the second conductor between a cut face of the third conductor and the first conductor.
The first to third conductors form a folded structure with the second conductor (plug) serving as a joint. This structure enables peripheral circuits including a redundancy circuit related to the first to third conductors to be arranged on one side thereof, thereby minimizing the semiconductor device. The fuse, which includes the second and third conductors, is easy to blow by irradiating a contact face between the second and third conductors with a laser beam. Since the fuse needs a small space, a plurality of fuses may be arranged in two rows. In this case, peripheral circuits related to the fuses are arranged on one side of each row. The fuses in two rows may be zigzagged, to reduce an area where the fuses are arranged.
When a fuse in the semiconductor device of the first aspect is blown, the first conductor under the first insulating film is exposed as a fourth conductor, and a cut face of the third conductor under the second insulating film is exposed as a sixth conductor, which is properly isolated from the fourth conductor. Below the thin area of the second insulating film, the first insulating film has an opening to expose the fourth conductor. Above the opening of the first insulating film, the second insulating film has an opening to expose the sixth conductor. Consequently, a distance corresponding to the length of the second conductor is secured as an isolation distance between the fourth and sixth conductors.
According to the first aspect, the fuse is mainly made of aluminum or copper. The aluminum has low melting and boiling points, and therefore, is blown at a low temperature. The copper has low electric resistance to form a thin fuse having a small volume to be blown.
A second aspect of the present invention provides a semiconductor integrated circuit having a latch circuit and first and second fuses arranged on a semiconductor substrate. The latch circuit holds data in response to a voltage applied to an input terminal. The first fuse has a first terminal connected to the input terminal of the latch circuit and is blown by a laser beam. The second fuse is formed under the first fuse and has a first terminal connected to a second terminal of the first fuse and a second terminal set at a predetermined voltage. The first fuse corresponds to the first or third conductor of the first aspect, and the second fuse corresponds to the second conductor. The fuses are properly blown by a laser beam.
A third aspect of the present invention provides a method of manufacturing a semiconductor device, having the steps of forming a first conductor, forming a first insulating layer and a columnar second conductor on the first conductor, forming a linear third conductor on the second conductor and first insulating film, forming a second insulating film over the third conductor and first insulating film, forming a thin area in the second insulating film over the second conductor, testing whether or not the first and second conductors must be disconnected from each other, and if they must be disconnected from each other, emitting a laser beam toward a contact face between the first and second conductors. The third aspect is capable of employing standard multilayer processes to form the second and third conductors serving as fuses.